1. Field
Embodiments described herein relate generally to magnetic resonance imaging (MRI).
2. Description of the Related Art
MRI is an imaging method which magnetically excites nuclear spins of an object (a patient) set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation. The aforementioned MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a nuclear magnetic resonance (NMR) signal.
In MRI, an RF coil device corresponding to the imaging part is set on an object as a phased array coil and MR signals are detected by the RF coil device, in some cases. Generally, though sensitivity of a whole body coil built into an MRI apparatus is sufficiently uniform, sensitivity of respective coil elements inside RF coil devices each of which corresponds to a different imaging part is not so uniform as compared with the whole body coil. That is, even if the intensity of MR signals emitted from an object is uniform regardless of position, levels of detected signals become non-uniform and this degrades image quality.
Additionally, if an object is put in an imaging space where a static magnetic field with uniformity value higher than a predetermined value is formed, the uniformity of the static magnetic field is degraded due to difference in magnetic permeability between the inside of the object and its surrounding area.
Then, a prescan is performed before a main scan so as to determine conditions used in image reconstruction after the main scan and the imaging condition of the main scan based on the result of the prescan, in some cases. Note that, a “scan” means an operation acquiring MR signals and does not include image reconstruction.
As a prescan, for example, there are the following sequences: magnetic field measuring sequence for calculating an offset magnetic field (see, for example, Patent Document 1), b) a sequence for generating the sensitivity distribution map of the respective coil elements of an RF coil device (see, for example, Patent Document 2), and (c) a sequence for calculating a corrected value of the center frequency of RF pulses in the main scan (see, for example, Patent Document 3).
[Patent Document 1] Japanese Publication of Patent Application No. 2011-152348
[Patent Document 2] Japanese Publication of Patent Application No. 2005-237703
[Patent Document 3] Japanese Publication of Patent Application No. 2009-34152
The aforementioned offset magnetic field is used as an index of correction for uniformizing the static magnetic field of the main scan after the object is set in the imaging space. Additionally, the aforementioned sensitivity distribution map is used to correct accident error of a luminance level of each pixel caused by non-uniform sensitivity of the phased array coil in image processing after the main scan and image reconstruction processing, for example.
Because the results of prescans have a large effect on the image quality of images obtained by the main scan, it is desired to perform a prescan under more appropriate conditions. However, conditions of a prescan are automatically set in a uniform manner regardless of the imaging conditions, or conditions of a prescan are manually set by a user in order for the conditions to become appropriate.
Therefore, MRI technology to appropriately set conditions of scans for calibration such as a prescan without manipulation by a user has been desired.